Processes for hydrogenation of organic substrates are well known. A particular class of organic substrates which it is desirable to hydrogenate is the class of oligomers and polymers and in particular of the subclass of elastomers. The polymer SBS (styrene-butadiene-styrene) has been commercially hydrogenated for some 25 to 30 years, and sold as a higher added value range of elastomers with improved stability.
Polymer SBS is soluble in organic solvents to give a highly viscous solution (so-called polymer cement) offering huge steric hindrance to catalysts. Accordingly hydrogenation processes to date have employed colloidal nickel/aluminium catalysts contacted at 80.degree. C. and 60 bar hydrogen with the dissolved elastomer. Unfortunately the catalyst system is prone to formation of metal residues during the reaction stage, which remain in the polymer cement and contaminate the product. Accordingly a subsequent stage is usually employed for the removal of metal residues. Efforts to date have concentrated on minimizing the level of ash in the product. Nevertheless metal residue removal stages remain singularly responsible as the major contributor to capital costs of these commercial systems.
Previous attempts to employ catalysts which are not prone to residue formation have been less than successful. Heterogeneous catalyst systems were found to show low activities which had to be compensated by increased reaction temperatures in the order of 200.degree. C. Unfortunately, reaction under these conditions resulted in deterioration of the molecular weight of the product, and in the contamination of the product with metal resulting from catalyst attrition. In U.S. Pat. No. 5,378,767 is described a process comprising a fixed bed hydrogenation of polydiene polymers of MW of up to 10,000, wherein the fixed bed comprises platinum, palladium or a mixture of the two supported on an alpha alumina support mm-size particles in fixed bed at elevated temperatures of the order of 200.degree. C. with high conversion. However it was noted that some polymer degradation was observed due to the severe conditions employed.
Heterogeneous processes are known for the hydrogenation of low MW organic substrates. The heterogeneous process of EP 0 233 642 aims to improve selectivity of hydrogenation of low molecular weight, non-viscous substrates, comprising vegetable oils of MW in the range 600 to 1400 and viscosity typically in the region of or less than 10 cps at an operating temperature in the region of 135.degree. C. The oils which are thought to become trapped in fixed bed catalyst structures, saturated with hydrogen and fully hydrogenated can be instead partially hydrogenated with catalysts comprising foils or honeycombs, which provide easy exit of the intermediate partially hydrogenated product. Accordingly this publication teaches only that foil and honeycomb catalysts can give the improvement in selectivity of conversion of the low molecular weight, low viscosity feed but gives no information about their suitability for converting higher MW (of the order of .times.100) higher viscosity (of the order of .times.100) substrates.
Organic substrates such as oligomers and polymers and in particular the sub class of elastomers are moreover sensitive to non-selective hydrogenation resulting in deterioration of physical and chemical properties and the like.
From U.S. Pat. Nos. 5,028,665 and 5,110,779 there is known a heterogeneous catalyst comprising a Group VIII metal and a porous support, wherein the porous support is characterised by a pore size distribution such that at least 95% of the pore volume is defined by pores having diameters greater than 450 angstroms and the ratio of metal surface area to carrier surface area is in the range from about 0.07 to 0.75:1. These processes, however, use catalyst particles having relatively small particle sizes of 10 to 20 microns that are slurried in the polymer solution and although they may be separated from the solution of the hydrogenated polymer by conventional methods such as precipitation or centrifugal separation of filtration, separation of the catalyst and its fines from the highly viscous polymer solution will be no minor task and hardly perfect.
Finally processes are known for the hydrogenative conversion of organic substrates to obtain further useful products having different characteristic chemical and physical properties. For example it is known to convert polyketones to polyalcohols with use of conventional hydrogenation catalysts.
Accordingly there is a need for a hydrogenation process for organic substrates which is capable of hydrogenation in selective manner of a wide range of substrates, without need for removal of residues of the hydrogenation metal from the product, and without deterioration of the molecular weight of the product or contamination by catalyst attrition or the like, and which is adapted for commercial operation.